This invention is related to a method and to a device for adjusting a camera with an image sensor for taking a picture and with an objective for imaging an object on the image sensor.
During the use of cameras, for example in the scope of an industrial production for the monitoring, the control and the regulation of production procedures, it is necessary to adjust the camera. For this, the adjustment comprises the image sharpness setting of the optical imaging by appropriate corrective measures, as e.g. the change of the image width or object width of the optical imaging, or the adjustment of the objective, as well as the alignment of the position of the image field, which must correspond to the required visual field on the object, and which is adjusted by the position or by the alignment of the image sensor. These adjustment measures must be performed on the camera directly and on site.
In case of camera systems with image sensors, a video signal or a video-similar signal is often available for this purpose, allowing to display the image taken by the image sensor on a locally connected video monitor, so that the sharpness and the position of the picture taken can be controlled using the display of this monitor. This method, however, has the disadvantage that the camera must be equipped with a sophisticated electronic system for generating the video signal required on site, and that the camera can only be adjusted if a connected monitor is available for this purpose.
Another known possibility to check and to adjust the sharpness and/or the image field, even if no video signal or monitor is available, is to equip the camera with an autofocussing device which adjusts the correct sharpness automatically, making a manual adjustment of the sharpness superfluous, as well as including an additional viewfinder in the camera allowing to view the image field. This solution has the disadvantage that an autofocussing device is technically sophisticated, as the distance adjustment, in many application cases, does not have to be performed often, e.g. only during the initial set-up, or in case of maintenance and repair measures. The viewfinder used for this has the disadvantage that the image field cannot be recognized sufficiently in many cases, in particular if the object width varies, and that the corresponding space conditions do not always allow to view the viewfinder with the human eye.
For this reason, projection devices as to the state-of the-art are known which project an adjustment mark onto the object plane with a projection device; the sharpness of the image and/or the image field can be controlled and adjusted with this adjustment mark. The adjustment marks are made visible by an appropriate screen, e.g. a simple piece of white paper.
A known projection system of this type uses two projection devices independent from the imaging system for the image sensor, each of them projecting a circle onto the object plane. Both circles are identical in diameter and the projection is performed with an angle slightly deviating from parallelism. The projection devices are adjusted in a way that the two projected circles unite to form a single circle, in case of a correct distance between the image sensor and the object to be taken, i.e. in case of optimum image sharpness. A third projection device is arranged, projecting a third circle, roughly indicating the size and the position of the visual field, i.e. the range of the object displayed on the image field of the image sensor.
Such known systems with projection devices are technically sophisticated, as at least one further imaging optical system for the adjustment projection devices is necessary besides the underlying imaging optical system of the camera. Furthermore, in case of a possible readjustment or change of the optical system, it is necessary to perform a complete new adjustment of the projection device.
An optical system for which an automatic focussing process is performed is known from the document EP 0080340 B1. Here, the optical system projects light onto the object, the object reflects the light and the optical system captures the light again; for focusing purposes, the light is displayed on an image sensor which is arranged in the image plane of the objective. Adjustment and setting on site using the human eye is not possible here without using technical equipment for generating an image, captured by the image sensor for adjustment purposes. From document DE-OS 2413155 a comparable device is known which uses two image sensors.
A projection device for adjusting the image field is known from document U.S. Pat. No. 4,899,188, however, it does not solve the problem of the focusing of a reproduction device.
An image field recognition device is known from the document GB 2257800 A. For this, laser systems are used the light of which does not pass the camera objective. Thus, a sharpness control in not possible.